1. Technical Field
The present specification describes a charging control circuit, and more particularly, a charging control circuit for charging a secondary battery.
2. Discussion of the Background
Portable devices using a secondary battery as a power supply, such as cellular phones, are now widely used. Such portable devices use a lithium-ion battery having reduced size and weight and a large capacity. However, the lithium-ion battery does not have a mechanism for preventing overcharging, and thereby may pose problems of durability and safety when overcharged.
To address this problem, related-art lithium-ion batteries used as secondary batteries are charged by constant current charging, and then charged by constant voltage charging after a voltage of the secondary battery reaches a predetermined voltage, so that the voltage of the secondary battery does not exceed the predetermined voltage.
FIG. 1 is a circuit diagram of a related-art charging circuit for charging a secondary battery. In a charging circuit 120, when a secondary battery BatR has a low voltage and a voltage of a non-inverting input terminal of an operational amplifier circuit 121 for constant voltage control is not higher than a reference voltage Vref1 generated by a reference voltage generation circuit 123, an output terminal of the operational amplifier circuit 121 has a low level voltage. In this state, an operational amplifier circuit 122 for constant current control controls a charging current to the secondary battery BatR. For example, the operational amplifier circuit 122 controls a gate voltage of a driver transistor M101 so that a voltage decreased by a resistor RD for detecting the charging current is equal to a reference voltage Vref2 generated by the reference voltage generation circuit 123. Thus, a charging current ic becomes a constant current that can be represented by the formula ic=Vref2/rd, in which rd represents a resistance value of the resistor RD.
When the voltage of the secondary battery BatR increases and the voltage of the non-inverting input terminal of the operational amplifier circuit 121 for constant voltage charging reaches the reference voltage Vref1, an output voltage of the operational amplifier circuit 121 increases and the operational amplifier circuit 121 controls a charging current. The operational amplifier circuit 121 controls the gate voltage of the driver transistor M101 to decrease a drain current (e.g., a charging current) of the driver transistor M101 so that the voltage of the secondary battery BatR does not increase further. Accordingly, the voltage decreased by the resistor RD is not higher than the reference voltage Vref2, and an output terminal of the operational amplifier circuit 122 for constant current charging has a low level voltage. Consequently, constant voltage charging is performed at a voltage V that can be represented by the formula V=Vref1×(r121+r122)/r121, in which r121 and r122 represent resistance values of resistors R121 and R122, respectively. At the voltage V, the voltage of the non-inverting input terminal of the operational amplifier circuit 121 is equal to the reference voltage Vref1.
There is an increasing demand for an ability to charge the secondary battery using a USB (universal serial bus) Vbus on a personal computer. However, generally lithium-ion batteries are charged at a charging voltage that ranges from 4.1 V to 4.2 V, whereas USB Vbus voltages are nominally 5.0 V. Therefore, in order to use a Vbus as a power supply for charging the lithium-ion battery, a voltage difference between the Vbus voltage and the secondary battery voltage had to be used effectively.
To address this, a driver transistor and a mirror transistor instead of the resistor RD may be used to detect a charging current. For example, the driver transistor performs charging control while the mirror transistor generates and outputs a proportional current proportional to a current output by the driver transistor, so as to detect a current of the mirror transistor.
However, a ratio between a charging current and a detected current may fluctuate depending on the voltage difference between a power supply voltage Vcc and a voltage of the secondary battery BatR. For example, when the secondary battery BatR has a low voltage, the difference between the power supply voltage Vcc and the voltage of the secondary battery BatR is great, thereby possibly increasing the charging current. By contrast, as the voltage of the secondary battery BatR increases, the charging current may decrease.
In order to stabilize a ratio of the proportional current, there is a technology to cause a voltage between drain and source of the driver transistor to be equal to a voltage between drain and source of the mirror transistor. However, in a desaturation region in which the voltage between drain and source is lower than −1 V, a drain current of the driver transistor may decrease substantially, and therefore a ratio between the drain current of the driver transistor and a drain current of the mirror transistor may decrease.
Consequently, even when the voltage between drain and source of the driver transistor is equal to the voltage between drain and source of the mirror transistor, the charging current may decrease as charging progresses, thereby possibly lengthening charging time.